A Really Big (but Extinct) Rodent

Published: 07/01/2010

Guinea pigs make popular pets nowadays. Eight million years ago, however, it would have been hard to find a cage large enough to hold one.

Back then, a South American rodent called Phoberomys pattersoni grew to be as big as a bison, say researchers who recently found some new Phoberomys fossils in northwestern Venezuela. Analyses of the 8-million-year-old fossils suggest that the rodents could reach a weight of 740 kilograms, or more than 1,600 pounds.

Phoberomys belongs to the caviomorph family of rodents, related to modern-day guinea pigs, chinchillas, and capybaras, which at 50 kilograms, are today’s largest rodents. Researchers first learned about Phoberomys in 1980, but bone and tooth fossils weren’t complete enough for them to estimate the animal’s size.

The new fossil finds suggest that the enormous creatures could sit on their hind legs like modern rodents, using their front paws to handle objects. The researchers also found crocodile, fish, and freshwater turtle remains near the Phoberomys fossils, suggesting that the rodents probably spent part of their time in water eating aquatic grasses.

The researchers speculate that Phoberomys was able to get so huge because there weren’t any grazing animals such as horses or cows to compete with them. The rodents disappeared when ferocious predators arrived on the continent.

For us, their extinction is probably a good thing. It might be tough to clean up after your cat if it happened to drag one of these things into the house!—E. Sohn